In electronics, a limiter circuit is a circuit that allows radio frequency (RF) signals below a specified input power to pass while attenuating the peaks of stronger signals that exceed the specified input power. Limiter circuits are commonly used, for example, in radio detection and ranging (radar) systems, to protect receivers from high power transmitters that use the same antennas.
FIGS. 1A and 1B illustrate a pair of limiter circuits A, B associated with commercially available devices that use PIN diodes. If the power of a signal at an input 14 is within range, the signal propagates to the output 16 of the limiter circuit A, B unattenuated, with PIN diodes 2, 4 exhibiting a small capacitance. As the power of the signal increases and the PIN diodes 2, 4 detect a threshold voltage present in the forward direction, the PIN diodes 2, 4 begin to conduct. In the limiter circuit of FIG. 1A, the positive polarity component of an RF signal received at the input 14 is shunted to ground when a single PIN diode 2 detects a voltage exceeding a threshold. A return path is provided for the PIN diode 2 via an inductor 6 connected to ground. In the limiter circuit of FIG. 1B, the positive polarity component of an RF signal received at the input 14 is shunted to ground when a first PIN diode 2 detects a voltage exceeding a threshold and the negative polarity component of the RF signal received at the input 14 is shunted to ground when a second PIN diode 4 substituted for the return path detects a voltage exceeding a threshold.
FIG. 1C is an example plot approximating power curves for limiter circuits such as shown in FIGS. 1A and 1B. As reflected in the power curves, the limiter circuits begin limiting at relatively low powers. Further, the power curves are frequency dependent. A PIN diode stores a relatively large charge in an intrinsic region of the PIN diode. The charge is removable at low frequencies to allow the PIN diode to turn off. The PIN diode, therefore, obeys the standard diode equation for low frequency signals. At higher frequencies, there is not enough time to store the charge and the PIN diode does not turn ON, allowing high power, high frequency signals to pass through the limiter circuit unattenuated.
The power response characteristics of limiter circuits as shown in FIG. 1A-1C make such limiter circuits generally undesirable for use with high precision signal measurement devices, particularly signal measurement devices intended for use measuring very high frequency signals, and/or which can be exposed to high power, very high frequency signals from an input source.